1. Field of the Invention
This invention relates to digital telecommunication systems and more particularly to an in-service line monitoring technique to detect and isolate marginal and failed digital repeatered sections in a digital transmission system. This patent application is related to application Ser. No. 777,802, filed Sept. 19, 1985, and is assigned to the same assignee as the present application.
2. Background Description
Digital error detectors, spaced along a digital repeatered transmission system, usually detect errors by detecting coding violations in the digital line signal. The number of coding violations in any given period of time can be and usually is translated into a bit error rate (BER). The BER is a measure of error density, i.e. number of errors/the total number of bits measured. For example, a BER of 1.times.10.sup.-7 would indicate an error every 10 million bits measured. The BER is a very meaningful measure of the performance of a digital transmission system. And, it is an excellent maintenance and diagnostic tool since anything which may affect the quality of the transmitted signal will affect the BER.
Error detectors monitor the operation of each span in a digital system by checking the BER. Such detectors usually have an internal alarm threshold level setting (often 1.times.10.sup.-5 or 1.times.10.sup.-4). When this BER is exceeded, an alarm indicative of unacceptable transmission is sent back to a master station over a return path service channel of the transmission system. In fully redundant systems, the alarm signal causes the system to take appropriate action automatically by switching the effected traffic to a spare link or by otherwise removing the failed span from service.
While this technique is effective, there are inherent drawbacks. The alarm condition is reported only after the system transmission quality deteriorates beyond acceptable limits. Thus, the BER information transmitted prior to the alarm signal may itself be faulty unless the failure has occurred in only one direction. Further, once the system deteriorates to unacceptable limits, maintenance personnel must proceed step-by-step to locate the specific link or maintenance section in the repeated system that has failed. It would be preferable to determine when the transmission path has deteriorated some amount less than catastropic failure and to then correct the problem or effect a transfer of the communication circuits before the system goes into alarm and shuts down. Merely lowering the alarm threshold level is a poor solution. Although it would detect a developing service problem, it would not be able to detect if the system deteriorates to the point of catastropic system failure. Thus it is an object of this invention to be able to detect when a complete system failure occurs and also to detect, prior to a path failure, when a system is deteriorating and to isolate that specific deteriorating link.